#include <bits/stdc++.h>
using namespace std;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	int k, day, dayCount = 0, ans = 0, s, t;
	int &coin = dayCount;
	cin >> k;
	for (day = dayCount = 1; day <= k; ++dayCount)
	{
		s = day;
		t = day + dayCount;
		if (t <= k)
		{
			ans += dayCount * coin;
			day = t;
		}
		else
		{
			ans += coin * (k - s + 1);
			break;
		}
	}
	cout << ans;
	return 0;
}
